It’s become more common for professional cyclists to share power meter files from races and training, providing a new insight into the physical demands of the world’s top races. Over his long career, the exploits of Jens Voigt have reached almost mythical status.

Earlier this season, Jens posted his SRM file from stage 18 of the Tour de France. He broke away early on the stage, which involved climbing Alpe d’Huez twice! Analysing these power files provides an interesting opportunity to compare our own performance relative to professional riders. In his Stage 18 escape, Jens spent a lot of time driving the breakaway group, recording an average power of 4.1 watts/kg over 5:14hrs. In comparison, during a recent sportive, I produced an average power of 3.4watts/kg watts over 4:47 hours. Gaining further into Jens’ file, we can see that he recorded a massive 5800 kilojoules during the 172.5km stage. In a small 4 man group, I produced 3800 kilojoules of mechanical work over 160km. SRM recently posed a good question in a tweet from their official account:

Many pro-cyclists, coaches and scientists use kilojoules as a metric in analysing training, guiding nutritional strategies, prescribing sessions and controling experiments. In fact, some pro cyclist have become renowned for their predilection for weighing their food in an attempt to balance energy intake with expenditure, as recorded by their power meters. As the price of power meters drops and increasing numbers of cyclists begin to measure and record their power output, I expect awareness and discussion of the use of kilojoules, as a training measure, to grow.

 How Scientists May Use Kilojoules

In experiments, scientists sometimes ‘prescribe’ a protocol which requires the participants to expend a set number of kilojoules, such as in the Gibala study in the ‘Time Poor Cyclist‘ articles. This approach ensures that all the subjects have done the same amount of mechanical work, which aids comparison and improves the reliability and validity of the study’s results. For example, 2 people may ride a 40km time trial, but depending on the intensity they ride at, the amount of mechanical work to complete this TT, and therefore the energy expenditure, could be very different. Setting a kilojoule target for subjects is more consistent and repeatable. 

For Cyclists & Coaches

The amount of mechanical work a cyclist has done can be used in helping to determine their nutritional strategy. You may notice that I’ve already emphasized mechanical work a number of times in this article. During training and racing, a power meter can record how many kilojoules of work have resulted in the pedals turning. However, the human body can only operate at around 25% efficiency. Consequently, doing 1kj of mechanical work on the bike actually requires the body to expend 4kj. Therefore, producing 3800kj of mechanical work during my sportive ride actually required my body to expend around 15,200kj.

So How Much Do I Need To Eat?

Eating for performance requires many considerations. Athletes must eat to meet the demands of:

  • Living
  • Training
  • Adapting to training stimulus
  • Maximising exercise capacity for competition
  • Protecting immune function
  • Maintaining health
  • Achieving body composition targets
  • Integrating socially!

The quality and nutrient density of an athlete’s food are vital to achieving these objectives in addition to controlling consumption relative to the athlete’s overall energy expenditure. However, in the context of training and racing with a power meter, it’s the energy in food that we a primarily concerned with.

An athlete’s energy expenditure may be described as comprising 3 components:

1. Basal metabolic rate: The amount of energy required to stay alive makes up about 60-80% of total energy expenditure and is related to the amount of active tissue that the individual has.

2. Thermic effect’ of feeding: The metabolic cost of the body processing food.

3. Normal daily activity, training/competition: This is usually the most variable component in the energy ‘mix’.

(Adapted from Broad & Cox, 2008)

Deciding how much energy we need to consume requires us to consider how much energy has been expended across all three components. In part 2 of this blog, we’ll explore:

  • Why all calories are not created equal
  • The relationship between calories and kilojoules
  • How watts recorded by a power meter relate to the energy we expend
  • What this could mean for your nutritional strategy, training and competition
  • Thoughts on nutrition, training and timing, which may challenge the received wisdom of endurance athletes!